Papers by Author: Jae Hwa Lee

Abstract: Ti-51Ni(at%) and Ti-40Ni-10Cu(at%) alloy wires with diameters of 0.3mm, 0.5mm and 0.7mm were prepared by drawing the alloy ingots fabricated by vacuum induction melting. Heating rates of the wires were investigated by measuring changes in temperatures of them while applying currents in the range of 1 A and 6 A to them and cooling rates were investigated by measuring changes in temperatures of them after cutting currents. Heating rate increased with increasing the amount of current, while cooling rate was kept constant. Both heating rate and cooling rate increased with decreasing diameter of wire. This suggested that high amount of current and small wire diameter were required for high heating and cooling rate. Comparing Ti-50Ni alloy wires with Ti-40Ni-10Cu alloy wires, heating rates of the latter was faster than that of the former, although cooling rates were almost same. This suggested that Ti-40Ni-10Cu alloy wires is better than Ti-50Ni alloy wires for the applications requiring high actuating rates.

Abstract: Shape memory characteristics and superelasticity of an equiatomic Ti-Ni alloy with surface sulfide layers were investigated by means of thermal cycling tests under constant load, tensile tests, and scanning electron microscopy. Martensitic transformation start temperature (Ms) increased by sulfidation, which is ascribed to a compressive stress imposed by surface sulfide layers. Surface sulfide layers were found to make transformation elongation be small when their thickness was less than 5 ㎛. This is ascribed to the fact that the surface sulfide layers were not detached from substrates and constrained martensitic transformation. When thickness of the surface sulfide layers was 35 ㎛, transformation elongation was not affected by them because they were detached during transformation. Superelastic recovery decreased by the sulfide layers when their thickness was about 5 ㎛, while it did not change when the thickness was about 35 ㎛.

Abstract: Transformation behaviors and shape memory characteristics of Ti-45Ni-5Cu alloy ribbons fabricated by melt spinning were investigated by means of optical microscopy, differential scanning calorimetries(DSC), X-ray diffraction and thermal cycling tests under constant load. They depended largely on temperatures of liquid metal. The B2-B19-B19’ two-step transformation occurred in the ribbons fabricated with the liquid whose temperature was higher than 1723 K, while the B2-B19’ one-step transformation occurred in the ribbons with the liquid at 1673 K. The stabilization of the B19 martensite in Ti-45Ni-5Cu alloy ribbons was ascribed to the high density of dislocations which made strong resistance to large lattice deformation associated with a formation of the B19’ martensite.